3.0L taurus Q
 

I know, its not a truck, but this forum has been helpful in the past, and I didn't want to join another forum to ask.

2000 taurus 3.0L vulcan engine 145K miles. Starts, and runs fine.

the problem is the idle. it should idle around 900 rpms, but it drops down to 400-500 every 20 seconds or so. Does it wheather is cold or warm.

I was thinking IAC, but it only does this with the defroster on. Turn the defroster off, and it will idle normally all day. When its really Cold and the car has been sitting (over night) i have to pump the gas pedal to get it to start.

Any ideas?

i would definatly check into your ac...my truck did this when my fan clutch went out and i would start my truck and the ac condensor would overheat and make me ac kick in and out and would do this intill it for warmed up and cooling good.......as when you turn the defrost on it kicks the compressor on..... sometimes when mine would start it sounded like the battery was low

With 145k, it wouldn't be a bad idea to clean the throttle body and IAC.